Films of an aromatic polyimide obtained by polymerization-cyclization (i.e., polymerization-imidization) reaction of an aromatic tetracarboxylic anhydride and an aromatic diamine are excellent in heat resistance and chemical stability, so that they have been paid much attention as excellent industrial materials applicable to various uses. Particularly, a film of an aromatic polyimide obtained by polymerization reaction of 3,4,3',4'-biphenyltetracarboxylic dianhydride and p-phenylenediamine is known to have remarkably high heat resistance and chemical stability.
For the preparation of aromatic polyimide films, there has been conventionally utilized a process comprising the steps of polymerizing an aromatic tetracarboxylic dianhydride and an aromatic diamine in a polar organic solvent to prepare a solution of a polyamide acid (also referred to as "polyamic acid"), then casting the polyamide acid solution on a support in the form of a film, heating the filmy cast solution at a temperature of 80.degree. to 200.degree. C. to obtain a self-supportable film, then peeling the self-supportable film from the support, and subjecting the film to heating-cyclization (ring closure) by, for example, further heating the film at a temperature of not lower than 300.degree. C. This process utilizing the heating-cyclization is simple and industrially advantageous, but has a problem that an amide acid bond of the polyamide acid is apt to be hydrolyzed owing to water produced in the cyclization reaction (imidization reaction) to cause reduction of molecular weight of the produced polymer, resulting in deterioration of physical properties of the resultant polyimide film.
For avoiding such problems as the above-mentioned deterioration of physical properties of the resultant film, there has been already developed a process using an imidization agent, namely, chemical imidization process. In that process, the imidization agent is added to the polyamide acid solution. In the presence of the imidization agent, hydrolysis of the amide acid bond is prevented and the cyclization (imidization) reaction can be performed by heat treatment at low temperatures, to obtain a polyimide film. As the imidization agent employable for this purpose, tertiary amines such as pyridine are conventionally known as described in "Polyamide Resin Handbook", edited by Osamu Fukumoto, published by Nikkan Kogyo Shim-bunsha, 1988, pp. 533-534. Pyridine is available at a low cost, so that it can be said that employment of pyridine is industrially advantageous. However, in the process using pyridine, it is essential to use a lower carboxylic acid anhydride such as acetic anhydride in combination therewith. Further, the pyridine is required to be used in an amount of not smaller than 0.5 mole per 1 mole of the amide acid bond of the polyamide acid. For these reasons, the employment of pyridine is not always advantageous in industrial use (see: Japanese Patent Provisional Publication No. 59(1984)-223725). Especially in the case of adding a heretofore employed tertiary amine and a lower carboxylic acid anhydride in combination to the polyamide acid solution for the purpose of chemical imidization, the solution is inavoidably converted into a gel in a short period of time, and thereby the subsequent essential steps for preparing a film such as a step of casting the solution is not smoothly conducted.
For coping with the problem of gelation, the aforementioned Japanese Patent Provisional Publication No. 59(1984)-223725 proposes that imidazole, benzimidazole or a derivative thereof be employed as a chemical imidization agent, without using the carboxylic acid anhydride in combination. However, problems still remain in this process. That is, imidazole, benzimidazole and derivatives thereof are relatively high in cost, and additionally, they are necessarily used in an amount of not less than 2 moles per 1 mole of the amide acid bond of the polyamide acid in order to perform imidization reaction so efficiently that the process is industrially advantageous.